A proof-of-concept hybrid magnetometer is presented, which
simultaneously operates as both a fluxgate and a search coil, allowing it to
sense the magnetic field from DC to 2 kHz using a single sensor.
Historically, such measurements would normally require two dedicated
instruments, and each would typically require deployment on its own dedicated
boom as the instruments mutually interfere. A racetrack fluxgate core
combined with a long solenoidal sense winding is shown to be moderately
effective as a search coil magnetometer, and the search coil effect can be
captured without introducing significant hardware complexity beyond what is
already present in a typical fluxgate instrument. Several methods of
optimising the search coil action of the hybrid instrument are compared with
the best method providing sensitivity and noise
performance between comparably sized traditional air-core and solid-core
search coil instruments. This hybrid sensor topology should miniaturise to
platforms such as CubeSats for which multiple boom-mounted instruments are
generally impractical, so a single hybrid instrument providing modest, but
scientifically useful, sensitivity from DC to kHz frequencies would be
beneficial.

Space science missions (e.g. Angelopoulos, 2009; Kessel et al., 2013) often
need two magnetic field instruments – a search coil magnetometer for high
(≥∼10 Hz) frequencies (Fig. 1a) and a fluxgate for the static field
and low frequencies (Fig. 1b). These two instruments will interfere with each
other if they are located too close together. The search coil can detect the
drive signal in the fluxgate magnetometer, and the ferromagnetic core
typically used for magnetic gain in the search coil distorts the field
measured by the fluxgate. The two instruments are therefore frequently
mounted on separate deployable booms, which affects spacecraft control and can
impinge on the field of view of other instruments.

Search coil magnetometers and fluxgate magnetometers (Hospodarsky, 2016;
Primdahl, 1979) both sense the local magnetic field through the
electromagnetic force (EMF) induced by changing magnetic flux
Φ described by the generalised induction equation (Eq. 1)
for a coil of wire of N turns and area A, in a field H,
with a ferromagnetic core of relative permeability, μr:

Vi=ddtΦ=ddtNμ0μrA⋅H≈Nμ0μrA⋅dHdt(1)+Nμ0μrH⋅dAdt+Nμ0A⋅Hdμrdt.

Search coils measure the voltage induced due to the changing magnetic field
dHdHdtdt,
while fluxgates measure the voltage induced by changing the relative
permeability of a periodically saturating ferromagnetic core dμrdμrdtdt. Search coils typically
have no single dominant noise source, so compact space instruments tend to
use a long ferromagnetic core for magnetic gain and tens of thousands of
turns of wire to increase sensitivity. Fluxgate noise is typically dominated
by the magnetic noise of the ferromagnetic core, so only small, compact flat
windings of a few hundred turns are required to avoid the coil being a
significant noise source. This paper describes a proof-of-concept hybrid
magnetometer which explores whether both the search coil and fluxgate sense
mechanisms can be simultaneously extracted from a common sense winding.

Several authors (Gordon et al., 1965; Hinnrichs et al., 2000, 2001; Ripka,
1990, 1993, 2000) have explored using long racetrack (oval) cores (Fig. 2d)
instead of circular (Fig. 2a) ring cores. The racetrack geometry mimics the
long cylindrical cores often used in search coil sensors (Fig. 2c) and is
compatible with similar long solenoidal sense windings (Fig. 2b). The
fluxgate action has been demonstrated to have bandwidth to at least 3 kHz
(Ioan et al., 1996; Miles et al., 2013; Primdahl et al., 1994). However,
since fluxgate sensitivity is essentially flat with frequency and search coil
gain tends to increase with frequency until the self-resonance of the coil,
search coil magnetometers tend to provide better sensitivity above a few tens
of Hz.

The complementarity of the two sensor technologies has led several authors to
investigate creating hybrid instruments which can exploit both the fluxgate
and the search coil effects. Two groups (Han et al., 2012; Shi et al., 2017)
are developing hybrid instruments by embedding a fluxgate within the hollow
core of a search coil and electrically fusing the outputs. This same technique
can be used for other DC magnetometer technologies, such as by embedding a
Hall sensor at the centre of a search coil core (Leroy et al., 2008). Ripka
et al. (1995) investigated whether the feedback coil in a fluxgate could also
be used as a search coil magnetometer. This paper extends that concept by
changing the geometry of the fluxgate sensor to mimic that of a search coil
such that both effects can be extracted from a common sense winding. Zhang et
al. (2010) used a similar sensor design but interleaved at ∼1 s
intervals and switched off the fluxgate excitation when operating the search
coil. The paper also examines the search coil signal extraction to see if it
can be improved by interleaving its capture with the fluxgate action on the
timescale of the excitation of the core.

Fluxgate magnetometer cores are periodically driven into magnetic saturation
to modulate the local magnetic field. The fluxgate signal is generated by the
core's changing permeability as it enters and exits magnetic saturation.
However, because the core is typically driven in resonance (Acuna, 1974) with
short-period high-amplitude current spikes, the core is in deep saturation
less than 25 % of the time. The solenoidal sense coil should act as a
solid-core search coil when the core is unsaturated, an air-core search coil
when the core is in saturation, and a fluxgate while the core is entering or
leaving saturation. If the saturation of the core and the sampling of the
sense coil are synchronised, the different sensor effects should be separable
using digital signal processing, allowing the sensitivity and noise floor of
the two effects to be compared. Utilising both the search coil and fluxgate
mechanisms may provide a way to combine the flat frequency response of the
fluxgate with the increasing gain with frequency of the search coil to
eventually span a range from DC to >10 kHz in a single instrument.

The racetrack cores presented here were produced as part of a larger project
to apply the theory of Narod (2014) to produce lower-noise fluxgate cores and
cores in non-traditional sizes and geometries (Miles et al., 2016). Most of
that work used 25.4 mm circular ring core (Fig. 2a) but a few racetrack cores
(Fig. 2d) were manufactured to explore the role of geometry in core noise.
The racetrack core is formed on an Inconel bobbin with an 82.5 mm long axis,
a 9.65 mm short axis, and a 1.65 mm channel on its outside surface
(Fig. 3). Three layers of a 100 µm foil are spiral wrapped into
the groove and attached with spot welds. The foil was manufactured by
successive cold-rolling of a 6.0–81.3 molybdenum–nickel Permalloy, which was
then coated with an insulating layer of magnesium oxide. The assembled bobbin
and foil were heat-treated for 4 h at 1150 ∘C in a reducing
atmosphere of 5 % hydrogen in 95 % argon.

The drive winding for the racetrack was 677 turns of AWG 32 enamelled wire
wound toroidally. The sense winding was arbitrarily chosen to be 2247
solenoidal turns of AWG 37 enamelled wire in four layers on a fused quartz tube
with an outer diameter of 15.4 mm. The length of the solenoidal winding was
roughly matched to the long axes of the racetrack core. A Permalloy rod of 7 mm
diameter and 95 mm length was used to compare the hybrid magnetometer
sensitivity to that of the same sense winding operated as a traditional
search coil with a solid core.

Figure 4 shows a block diagram of the set-up used to explore the hybrid
magnetometer concept. The magnetometer is operated open loop, without
magnetic feedback, to examine the natural response of the sensor. A
field-programmable gate array (FPGA) is used to send 2.5 kHz alternating
polarity power amplified (PA) current pulses (Idrive) into the
toroidal drive winding to periodically saturate the racetrack core. The
solenoidal sense winding is used in the short-circuit–current-output
configuration such that the changing magnetic flux due to the external field
(Hsense) experienced by the coil induces a current,
ISense. This current is converted to an equivalent voltage
(I/V), passed through a weak single-pole low-pass anti-alias filter with a −3 dB point at 5 kHz (LPF)
and then synchronously digitised (ADC) in phase with the drive by the FPGA.

Figure 4Schematic of the test set-up showing the driven sensor, analogue processing,
and parallel digital processing paths of common sampled data for fluxgate and
search coil reconstruction. Adapted from Miles et al. (2013).

Digital signal processing is used to separate the search coil and fluxgate
signals in the raw ADC stream. The fluxgate path follows a classic second
harmonic design; the data are band-pass filtered (BP) at the second harmonic,
run through a phase-sensitive inverter (PSI), integrated (INT), and then
decimated (DEC) to 400 sps. In the search coil path, the raw data are also
sliced into four separate streams corresponding to different phases of the
core magnetisation cycles (Slice) and each has a static offset removed
(Offset) and is then low-pass filtered (LPF) and decimated (DEC) to 2500 sps
search coil data streams.

Figure 5 shows a fast Fourier transform of the raw ADC samples captured from
the hybrid sensor driven as a fluxgate and exposed to a 100 nT sinusoidal
test signal. The test signal is visible at 100 Hz baseband (Fig. 5a) via the
search coil action. The fluxgate drive is clear at 2.5 kHz (Fig. 5b) and
7.5 kHz (Fig. 5d), and the fluxgate signal appears at the 5 kHz second harmonic
(Fig. 5c). The test signal appears in the inset through the fluxgate action
as the lower 4.9 kHz (Fig. 5e) and upper 5.1 kHz (Fig. 5f) sidebands
analogous to amplitude modulation radio encoding (Hinnrichs et al., 2001;
Miles et al., 2013). All signals are well separated in frequency space,
suggesting that they can likely be isolated and reconstructed using a
low-pass filter (red) for the search coil effect and a band-pass filter
(green) for the fluxgate.

The racetrack core was driven at 2.5 kHz using the resonant drive circuit
described in Miles et al. (2016). The current circulating in the toroidal
drive winding was measured using a 0.1 Ohm current snoop resistor and is
shown in Fig. 6a. Note that, due to the tuning of the resonant drive circuit,
the current pulses are engineered to have a high amplitude capable of pushing
the racetrack core into deep magnetic saturation but have a short duration.
This low-duty cycle reduces power consumption and has the core in deep
saturation less than 25 % of the time. The current output of the sense
winding, transformed into an equivalent voltage and anti-alias filtered, is
shown in Fig. 6b. Figure 6a and b were captured at high cadence using a bench-top
oscilloscope. The phase offset between the drive current pulses (Fig. 6a) and
the sensor output waveform (Fig. 6b) is believed to result from the RLC
behaviour of the sense coil and preamp.

An equivalent representative time series, captured through the ADC, is shown
in Fig. 6c. The ADC samples at 20 000 sps or 4 times faster than
required for a classic second harmonic direct digitised fluxgate. The
vertical lines show the trigger points of the ADC and are colour coded to
indicate how the data are “sliced” into eight time series corresponding to
the eight different phases in the magnetisation loop of the core as shown in
Table 1.

Slices 2/B and 6/B contain the majority of the fluxgate action produced by
the current pulses forcing the core into saturation, during which time the
sensor should function like an air-core search coil. Outside of deep
saturation the sensor should function like a traditional high-permeability
core search coil albeit with an unusually low-mass core. While the core is
recovering from saturation, it is likely still partially energised and will
exhibit Barkhausen noise. This stream of raw samples will be reconstructed
as both a fluxgate and search coil magnetometer below.

If it were somehow possible to sample the magnetic field at the core
directly then these saturation states could be accessed independently and
without interference. However, since the magnetic field is accessed via its
induction of current and/or voltage in the sense coil, the signal from the
different magnetic states must pass through the RLC filter formed by the
sense winding, the preamplifier, the anti-aliasing filter, and the analogue-to-digital converter. Each of these stages has a transfer function which will,
to varying degrees, blend the different saturation states unless the
combined phase delay of all the stages is below half a sample, which does not
appear to be the case here. Consequently, the various saturation states are
likely somewhat mixed and cannot be separated perfectly. Nevertheless, we
will explore treating the various saturation states separately to see if any
performance benefit can be gained.

Figure 7 shows the power spectral density (PSD) of the ADC stream with a
100 nT sinusoid at 100 Hz applied to the sensor. Black shows all ADC
samples and the colours show demodulation into eight separate streams
corresponding to Slices 1–8 in Fig. 6 and the eight phases of the racetrack
core magnetisation cycle.

The nine spectra have been normalised such that the 100 Hz test signal has a
common amplitude. As expected, the eight slices form four pairs corresponding
to the equivalent places in the positive and negative phases of the drive
signal. For the remainder of this paper the data will be sliced into
four streams made up as A (Slices 1 and 5), B (Slices 2 and 6), C (Slices 3 and
7), and D (Slices 4 and 8) as shown in Table 1. Slice B contains the majority
of the fluxgate action, so it seems plausible that, during the following
Slice C, the core is still somewhat energised. As the core returns to its
unsaturated state, Barkhausen jumps may be contributing to the high noise
floor observed in Slice C.

Note the presence of narrow spectral content at 20, 40, and 80 Hz in the
sliced spectra which are not present on the spectra generated from the raw
ADC samples. This is consistent with spectral folding, which would be expected
as the raw ADC samples are not band limited via an anti-aliasing filter
before the slicing as that would further mix the core saturation states. This
will have the practical effect of folding signals in the 2.5–10 kHz band,
mostly associated with the fluxgate mechanism, down into the search coil
bandwidth.

Figure 8 shows the steps in reconstructing the fluxgate signal. The fluxgate
action acts analogous to amplitude modulation (AM) radio encoding. A natural
signal at frequency fo is modulated to two sidebands around the second
harmonic of the drive signal 2f±fo. The raw ADC samples (Fig. 8a) are
therefore band-pass filtered (Fig. 8b) at the second harmonic of the drive
plus or minus the desired bandwidth of the fluxgate path (5000±500 Hz)
to isolate the fluxgate signal from the search coil signal at baseband and
the harmonics of the drive signal (2500 and 7500 Hz, respectively).

The band-pass-filtered data (Fig. 8b) manifest the 100 Hz magnetic test
signal as the envelope of the 5 kHz (2f) oscillation. This is demodulated
as marked in Fig. 8b. Every other extremum point is inverted and the
intermediary points are discarded. The intermediary points can be inverted
and included, although they introduce a significant 2f oscillation.
However, doing so was found to decrease the signal-to-noise ratio. The phase-sensitive inversion
restores the sinusoidal test signal (Fig. 8c), which is
then averaged and decimated to produce a 1000 sps fluxgate stream shown as
the circles in Fig. 8d.

Figure 9a shows that the sensitivity of the fluxgate path varies by less than
0.5 dB from DC to 100 Hz (normalised to 0 dB at DC) and 3 dB from DC to
500 Hz.

Figure 9b shows the power spectral density (PSD) noise floor of the fluxgate
path calculated with data taken while the sensor was in a three-layer Mumetal
magnetic shield. The core achieves a modest ∼15 pT Hz-1/2 at
1 Hz. The same foil in roughly the same quantity with the same heat
treatment achieves ∼6 pT Hz-1/2 at 1 Hz when wound onto a
25.4 mm circular ring core. Other authors (e.g. Hinnrichs et al., 2001;
Ripka, 1993) have achieved good noise and sensitivity using racetrack
geometry cores, so we interpret these results to suggest that something about
the manufacture or drive of the racetrack core presented here is not yet
optimised. This is beyond the scope of this paper and is the subject of
ongoing work.

Figure 10 shows the steps in the reconstruction of the search coil signals.
The raw ADC values (Fig. 10a) are low-pass filtered at 1000 Hz to remove the
fluxgate drive, creating the “all samples” trace in Fig. 10b, and then
decimated (Fig. 10c). The raw ADC samples are also sliced into four time
series (Slices A–D) corresponding to the four phases of the magnetisation
cycle of the core, low-pass filtered at 1000 Hz, and independently detrended
to remove the static offset caused by that phase of the magnetisation cycle
(Fig. 10b).

Figure 10Reconstruction of the search coil signal from (a) raw ADC
data to (b) sliced, low-pass filtered, and detrended and
(c) decimated to common cadence.

The five time series show the 500 Hz magnetic test signal at significantly
different amplitudes. This is consistent with the general trend shown in Fig.
7 in which each digitisation offset captures a different degree of core
saturation and hence a different magnetic gain. Figure 11a compares the
sensitivity (nT bit−1) as a function of frequency for each of the
hybrid search coil time series and for the same sense coil used as a
traditional search coil. The sense coil was operated as an air core, with a
solid Mumetal core, and with the racetrack core in unsaturated and
continually saturated states. As expected, all configurations perform
similarly above ∼2 kHz where the coil is expected to dominate.
At lower frequencies, the effect of the magnetic gain of the core becomes more significant and, below ∼100 Hz, the gain of the five hybrid search coil traces becomes unexpectedly
flat with frequency. The 500 Hz test signal was selected as it illustrates
the region where the magnetic gain of the core is significant to the search coil effect
– above frequencies where the fluxgate effect dominates and below frequencies where the
coil response dominates.

Between ∼200 and ∼2000 Hz the ordering and trend of the
sensitivities matches intuitive expectations. The air core and the
continually saturated racetrack core provide comparable gain. At the other
extreme, the unsaturated racetrack provides about half the gain of the solid
Mumetal rod. The five hybrid search coil reconstructions lie between these
with the unsliced data providing the lowest sensitivity.

The constant gain below ∼40 Hz for the hybrid search coil
reconstructions appears to be a frequency folding effect caused by
insufficient anti-aliasing before digitisation. The racetrack core is driven
at 2500 Hz, so the fluxgate modulation will manifest as sidebands around the
even harmonics at 5000, 10 000, 20 000 Hz, etc. A 1 Hz magnetic test
signal will therefore create a sideband at 20 001 Hz (eight harmonic). The
sensor is sampled at 20 000 sps, creating a 10 000 Hz Nyquist frequency.
Therefore, the 20 001 Hz sideband will frequency fold over the Nyquist
frequency and 0 Hz down to an aliased 1 Hz, which will dominate compared to
the small sensitivity of the search coil at low frequencies.

Figure 11(a) Comparison of frequency-dependent gain for various
hybrid and standard search coil configurations. (b) Comparison of
power spectral density noise floor for the various search coil
configurations. The constant gain and noise at low frequencies is believed to be an aliasing artefact.

A standard figure of merit for search coil performance is the power spectral
density noise floor of the instrument (Fig. 11b) using quiet data taken
inside a magnetic shield normalised by the frequency-dependent gain. As
expected, the solid Mumetal core provides the lowest noise, followed by the
unsaturated racetrack core. The air core and the continuously saturated
racetrack core provide the poorest result. The unsliced hybrid search coil
path performs between the solid- and air-core limits as expected. However,
the sliced hybrid search coil path performs significantly worse than the
air-core search coil above ∼500 Hz where the search coil gain can be accurately measured without
interference from the aliased fluxgate signal.

In the frequency range of ∼50–250 Hz, where the search coil gains can be
accurately measured but are not yet dominated by the sense coil, the gain
ratio between the unsaturated coil and the saturated coil is approximately
3.6. However, the ratio of noise levels at 100 Hz for the unsliced hybrid
search coil and the unsaturated racetrack core is larger, approximately 5.3.
The noise level of the hybrid search coil therefore cannot be entirely due to
lower magnetic gain. This implies the presence of an additional magnetic
noise source – potentially Barkhausen jumps due to the excitation of the
core.

Figure 12 compares the sensitivity and noise floor obtained from the hybrid
instrument using both the fluxgate reconstruction and the unsliced search
coil reconstruction. The fluxgate reconstruction essentially provides flat
sensitivity with frequency (Fig. 12a) and a superior noise floor at low
frequencies (Fig. 12b). As expected, the search coil reconstruction is much
more frequency dependent and provides modestly superior gain and noise above
∼1 kHz. Figure 12 also shows the performance of the same sense coil
operated as a traditional search coil with a solid Mumetal core to illustrate
how much search coil performance is sacrificed by operating it as a hybrid
sensor.

Figure 12(a) The fluxgate and search coil reconstructions of the
hybrid magnetometer data compared to the same sense winding operated as a
conventional search coil with a solid core. (a) Frequency-dependent
sensitivity and (b) instrumental noise floor. The flat response at
low frequencies for the search coil reconstruction of the hybrid magnetometer
is believed to be an aliasing artefact rather than true instrumental
response.

Developing a complete vector space-flight sensor based on the
proof-of-concept work described here will require addressing several design
issues: the addition of magnetic feedback to linearise and extend the range
of the fluxgate path, robust and highly orthogonal mounting of three sensor
axes, and likely the development of real-time processing firmware for the
search coil and fluxgate paths to avoid telemetering the raw digitised data.

All data presented here were taken with the instrument operating in open
loop. For most practical applications, magnetic feedback will be required to
extend the magnetic range, linearise the instrument, and potentially to
provide temperature compensation. In a primarily digital design, such as that
presented here, this magnetic feedback is typically accomplished using a
digital-to-analogue converter to force current either into the sense winding
or into a separate feedback winding and drive the field in the sensor towards
zero. The range of the instrument is then set by the maximum amount of
feedback current, the instrument's linearity is primarily dependent on the
feedback circuit, and feedback current can be made temperature sensitive to
compensate for changes in sensor geometry (Acuña et al., 1978; Miles et
al., 2017). Updates to the fluxgate feedback will undoubtedly impact the
search coil action, particularly in instruments with high-bandwidth feedback
used either to maximise instrument linearity or to track rapid changes in the magnetic field, such as on
spinning platforms. Characterisation, modelling, and compensation will be
required to correct for the dynamic behaviour of the magnetic feedback.
Although compensating for the dynamic behaviour of magnetic feedback is
required in most fluxgate magnetometers, the additional search coil action in
the hybrid magnetometer is likely to be particularly sensitive, which will
drive instrument complexity.

Fluxgate and search coil magnetometers build up a vector magnetic field
measurement from three nominally orthogonal projections. Small misalignment
of these axes can be mitigated using an orthogonality matrix correction.
However, a secondary effect which can be difficult to characterise and
calibrate out is the potential for cross-talk between the channels. The
presence of a ferromagnetic core and/or magnetic feedback are expected to
create cross-talk offsets in the same way as in standard fluxgate or
search coil sensors. A potentially trickier issue is whether the broader
bandwidth and higher sensitivity of the search coil path will cause it to
pick up transients from dynamical fluxgate behaviours such as, in an
offsetting instrument, updating the magnetic feedback in one of the other
sensor axes. Such effects will need to be carefully characterised and
potentially compensated for within the instrument.

The racetrack ring core and the glass-tube-based solenoidal winding used in
the proof-of-concept prototype have a mass of 6 and 46 g, respectively. Allowing for
some mass optimisation of the sense winding, three sensor axes, and a mount
to hold the axes orthogonal, we estimate that a complete vector hybrid sensor
based on the current racetrack cores would likely have a mass of ∼250–300 g and
extend ∼10 cm in each direction. Extrapolating from the current
laboratory electronics, a complete three-channel instrument would likely
consume ∼400–500 mW of power during normal operation.

A flight version of the hybrid magnetometer would likely need to implement
both fluxgate and search coil reconstruction in onboard processing. An
instrument with three channels sampling to 24 resolution at 20 000 sps
would require ∼1.4 Mbps of bandwidth for the forward loop, with some
lower but non-trivial bandwidth required to telemeter the magnetic feedback
applied to each sensor axis. For spacecraft applications with telemetry
constraints, this bandwidth could be significantly reduced by processing the
raw samples onboard the spacecraft into a lower-cadence fluxgate stream
(three channels of 24-bit fluxgate data at 100 sps requires ∼7 kbps)
and compressed spectral products, such as averaged spectra or filter banks
from the search coil path. This processing could be accomplished either in
real time in the FPGA or in software on an embedded computer in the
spacecraft.

The presented proof-of-concept instrument demonstrates how a racetrack sensor
can be simultaneously operated as both a fluxgate and a search coil
magnetometer. The combination of a racetrack fluxgate core and a long
solenoidal sense winding is modestly effective as a search coil magnetometer,
and the search coil effect can be captured without introducing significant
hardware complexity beyond what is already present in a typical fluxgate
instrument. The search coil action was found to have significantly different
magnetic gain at different phases in the magnetisation cycle of the core.
However, slicing the data to use only the phase with the highest magnetic gain
produced a higher instrumental noise floor than simply using all samples –
potentially due to the mixing of the saturation states by the combined
transfer function of the sense winding and electronics. The best hybrid
search coil reconstruction performed roughly 4 times better (6 dB), in
terms of power spectral density noise floor, than an air core with the same
sense winding and roughly 10 times worse (−10 dB) than the same sense
winding paired with a comparably sized solid Mumetal core. At present, the
noise floor and sensitivity of the hybrid search coil data are only modestly
better than the fluxgate data and only at frequencies above ∼1 kHz. In
principle, the fluxgate could be operated at higher (kHz) frequencies to
provide similar data with only a modest loss in sensitivity. However, since
it has been demonstrated that the fluxgate and search coil actions can be
extracted separately it may be possible to optimise the search coil behaviour
of the sensor to provide high-frequency sensitivity and a noise floor beyond
that which is possible with current fluxgate technology.

It seems likely that the number of turns in the sense winding would need to
be increased by roughly an order of magnitude, and the preamplifier gain
doubled or quadrupled, to provide a search coil performance comparable to that
of historical space-based instruments. This proof-of-concept instrument
achieves a noise floor of ∼3×10-5 nT2 Hz−1 at
1000 Hz compared to ∼5×10-10 nT2 Hz−1 for the
∼10 cm search coil on MMS and ∼3×10-11 nT2 Hz−1 for the ∼40 cm search coil on the Van
Allen Probes (Hospodarsky, 2016). The coil and preamplifier design will need
to be coupled with careful optimisation of drive frequency, the
self-resonance frequency of the coil, and digitiser resolution to exploit the
high search coil gain without exceeding the bandwidth of the analogue
electronics or the digitiser. In particular, the self-resonance of the coil
should likely be optimised to align with the baseband search coil bandwidth.
Combined with a higher-order low-pass anti-alias filter, this should improve
the search coil performance while preventing interference between the
search coil and fluxgate actions. It remains to be seen if the sensitivities
of the two effects in a hybrid instrument can simultaneously be made
operationally useful. It is also possible that with sufficiently high
instrumental sensitivity we will discover that the search coil action is
contaminated by Barkhausen noise, caused by periodically saturating the core,
at a level that limits the usefulness of the search coil data. This sensor
topology should miniaturise to platforms such as CubeSats for which multiple
boom-mounted instruments are generally impractical, so a single hybrid
instrument providing even modest sensitivity from DC to 10 kHz could be
beneficial.

The bandwidth of the search coil reconstruction is limited by the fluxgate
drive frequency. Moving to a thinner Permalloy foil should allow the
racetrack core to be efficiently driven at higher frequencies and enable a
wider search coil bandwidth. The fluxgate noise floor still seems to be
dominated by the magnetic noise of the core, so the number of turns in the
sense winding and its self-resonance could likely be optimised with minimal
impact on the fluxgate action as long as the bandwidth of the coupled
fluxgate drive does not saturate the instrument. Higher-resolution
digitisation (>16 bit) will likely be required to achieve a competitive
search coil noise level given the large analogue bandwidth necessary to
accommodate the raw fluxgate signal. Finally, the noise floor of the hybrid
search coil data implies the presence of an additional magnetic noise source,
which we speculate may be due to Barkhausen jumps due to excitation of the
core. This needs to be confirmed and, if this is the case, thickness,
number of layers, and heat treatment should be explored to see if the core
can be engineered to de-energise more rapidly.

DMM led the experiment, designed and built the experimental apparatus,
executed the experiment, analysed the data, and prepared the paper with
contributions from all authors. BBN created the magnetic material for the
racetrack sensor and designed the heat treatment of the racetrack cores. IRM and
DKM helped interpret the fluxgate behaviour of the hybrid instrument. DB
designed the racetrack core. GH helped interpret the search coil behaviour of
the hybrid instrument.

Work on the project was supported by the Canadian Space Agency under contract
9F063-140909/006/MTB. David M. Miles was supported by an NSERC PGSD graduate
scholarship, faculty start-up funding from the University of Iowa, and an Old
Gold Summer Fellowship from the University of Iowa College of Liberal Arts
and Sciences. Ian R. Mann is supported by a Discovery Grant from the Canadian
NSERC. The authors wish to thank Taryn Haluza-DeLay for manufacturing the
solenoidal sense winding and Zoë Dent for attaching the driving winding to
the racetrack core. Miroslaw Ciurzynski manufactured and heat-treated the
racetrack core and provided detailed comments on an early version of this
paper. Andy Kale provided comments on an early version of the experiment.
Joshua Larson provided comments on an early version of this paper. The
authors wish to thank Mark Moldwin and two anonymous reviewers for the
constructive comments on this paper, which significantly contributed to the
understanding and explanation of the impact of slicing the search coil data
and the origin of the noise floor in the hybrid search coil
reconstruction.

We present a proof-of-concept space-flight instrument that can simultaneously make measurements of both the low- and high-frequency local magnetic field. Previously, this would have required two separate instruments that would normally have had to be mounted separately on long deployable booms to keep them from interfering. This new hybrid instrument is expected to be particularly useful on extremely small spacecraft, such as CubeSats, which can only accommodate a few instruments.

We present a proof-of-concept space-flight instrument that can simultaneously make measurements...